The present invention relates to prosthetic joints, and more particularly to prosthetic joints used for total human knee joint replacement and which permit the knee joint to perform in an almost natural anatomical manner.
Medically, hinged knee joints are implanted when the tissues of the knee joint are grossly deteriorated from disease or other cause, particularly the loss of function of the four major ligaments of the knee: 2 cruciate ligaments and 2 collateral ligaments.
In the normal anatomical function of the healthy knee, the geometry of motion is complex. The knee is essentially a large knuckle joint, in fact, it is the largest joint in the body. The upper surface of the tibia provides two bearing surfaces which are essentially flat and essentially at right angles to the axis of the shaft of the tibia. The co-acting lower end of the femur consists of two large rounded ends called condyles. The condyles roll and slide on the two supporting surface of the tibia, called the tibial plateau. There is no congruency or symmetry in either joint between the two corresponding surfaces on the tibia and the two corresponding condyles. Normally, bone joint surface pressure loadings are disbursed favorably through cartilage covered articular surfaces, and the menisci and synovium contribute to adequate normal function. The two rolling and sliding actions, plus the result of forces and constraints put on the knee joint by muscles, ligaments, joint capsule, etc. result in a relative axial rotation between the two cooperating bones, the femur and the tibia in addition to the basic motion of flexion. If one sits with the leg fully extended in front of him, as he flexes the knee downward through 90 degrees, the tibia rotates inwardly, approximately 10 degrees. This axial rotation of the tibia is relatively small and not at all uniform per unit of flexion occurring mostly in the initial phases of flexion from the leg being fully extended. In general, hinge-type prosthetic joints do not incorporate this relative axial rotation and therefore do not closely simulate the natural action of the knee joint.
Furthermore, hinge type prosthetic knee joints have two problems associated with permanent anchorage in the bone. One, they are restrained against axial rotation of the tibia. This axial rigidity causes torque shock loads applied to the lower limb or foot to be resisted by the prosthetic fastening to the bone. Such fastening is most commonly achieved by use of a quick setting bone cement like polymethyl methacrylate, which materials do not well resist such shock loads. Two, a knee joint fitted with a hinge prosthesis has a limited range of flexion of perhaps 90.degree. to 120.degree.. If such a restored joint is subject to attempted flexing beyond its limit, the bending force can act on the bones to cause them to separate or distract. This occurrence can cause the hinge prosthesis to pull out of its cement bed. Therefore, these two factors can cause hinge type prostheses to come loose from their original implanted condition, which is a relatively frequent cause of failure for this type of prosthesis.
To reproduce a natural leg movement to the maximum possible extent, a prosthetic knee joint must provide two degrees of freedom, namely, bending or rotation of the tibia about an axis transverse to the shaft of the femur, which motion is called flexion, and rotation of the shaft of the tibia about its axis relative to the axis of the shaft of the femur. Furthermore, the prosthetic knee must accommodate the large stresses placed on the bearing surfaces of the prosthesis. These stresses can cause inadequate bearing surfaces to wear out. Obviously, wear of the bearing surfaces is undesirable, because movement of the prosthetic knee would be impaired and the debris resulting from wear would be harmful to the body.
To date, knee prostheses either do not provide rotation in more than one plane, or have bearing surfaces which tend to wear away. The development of a prosthetic joint involves a conundrum in that the greater the degree of success of the operative procedure, the more likely the chance of failure of the prosthesis. The preceding stems from the fact that the patient candidate for the prosthetic joint has impaired joint function and therefore limited mobility. If the implantation of an artificial joint reduces pain and improves mobility, the patient becomes more active. The greater the degree of improvement of joint function, the greater the probability of increased activity by the patient. Increased activity will be accompanied by increased load and motion on the joint, which combine to tend to cause mechanical failure of the artificial joint. This condition is particularly pronounced in the knee because of the complex geometry of motion and the large load forces involved.
To illustrate the magnitude of the forces acting within the knee joint, it is possible for a 200 lb. man in a half squat on one leg to impose a compressive load of about 900 lbs. at the joint surface between the femur and the tibia. This load is due only to static conditions. Dynamic loads can be significantly larger.
Considering only those prostheses permitting rotation in two planes, prostheses can be divided into at least three general classes.
A first class includes those knee prostheses in the form of inserts or modules, or resurfacing devices which replace the bicondylar joint of a human limb and are constructed to simulate the shape of that bicondylar joint. These prostheses attempt to imitate the natural motion of a joint by structuring the surfaces of the joint to imitate nature. However, these joints have high local loading stresses at selective points on the bearing surfaces and therefore tend to wear away at these selective points, thus resulting in degradation and ultimate failure of the prosthetic joint. As a result, such prostheses may have to be replaced after only a short period of use, depending of course on the degree of activity of the patient.
A second class of prostheses includes a ball and socket joint with means to limit motion to rotation in two planes. In such a joint, the means to prevent rotation in the third plane is commonly constructed so that unit bearing loads are high, and early bearing failure generally result when patient activity increases.
A third class of prostheses comprises a fixation stem implantable in a tibia and having an upstanding arm extending from the top of the fixation stem. The top of the upstanding arm may be in the form of a ball. The ball fits into a socket in the space between the condyloid members of the femoral prosthesis. In this construction, the condyloid prosthetic members of the prosthesis of the femur rotate and slide on the prosthetic surfaces of the tibia. This relative motion between the non-congruent prosthetic surfaces under the high bearing loads imposed by the function of the joint may lead to premature mechanical failure of the prosthetic bearing surfaces.
The preferred embodiment of the present invention overcomes the above-discussed disadvantages of the known art by providing a prosthetic joint in which the bones can undergo rotation in two planes, yet will not be subject to excessive unit bearing stresses. Bearing surfaces are arranged to be in contact over large areas of engagement so that the large forces exerted by reason of the function and mechanics of the knee are spread out over the entire area of the engagement. Mated bearing surfaces are of large area and are of congruent geometry. Therefore, local stresses (force per unit area) are reduced from those values found in the prior art wherein the bearing surfaces are not congruent. With the arrangement of the present invention, the prosthetic device will continue to function with a minimum of wear and simulate normal action of a human limb for long periods of time under conditions of normal patient activity.
The preferred embodiment substantially eliminates the transmission of shock torque loads through the prosthesis by virtue of allowing relative axial rotation between the bones. Such torque loads are transmitted through tendons, ligaments and other soft tissues covering the joint. In addition, this embodiment has no fixed coupling between the two bones and is designed to extend or distract should flexion beyond approximately 120.degree. cause the bones to move apart or move axially with respect to each other. Accordingly, the present invention overcomes the two principal factors contributing to loosening of hinge type prostheses.
Further, the preferred embodiment provides the very practical advantage of allowing the surgeon improved access to the posterior joint space during the surgical procedure.
One alternate form of the prosthesis includes a tibia stem component which is implantable directly in the tibia and which stem upper surface supports a bearing shoe. The bearing shoe is axially rotatable relative to the tibia and is confined by guide elements at the edge of the upper surface of the tibia stem component. This embodiment can be designed to allow the cruciate ligaments to remain intact. In this embodiment, the femoral and tibial components are not hingedly coupled.
A second alternate is similar to the above, and permits removal of a minimum of bone from the tibia.
Accordingly, it is an object of the present invention to provide a durable prosthetic joint which enables the limb to undergo approximate natural movement.
It is another object of the present invention to enable a pair of human bones joined by a prosthetic joint to undergo relative rotation in at least two planes and thereby closely simulate the natural action of the joint.
It is yet another object of the present invention to provide a prosthetic device having an improved bearing configuration.